Piezoelectric devices, including a piezoelectric vibration element, a piezoelectric oscillator and the like, have been widely used for small information equipment, such as HDD (hard disc drive), mobile computers, IC cards, and for mobile communications equipment such as cellular phones, car-phones, and paging systems, and piezoelectric gyro sensors, etc.
FIG. 10 is a schematic plan view illustrating an example of a piezoelectric vibration element conventionally used in the piezoelectric devices.
In the figure, a piezoelectric vibration element 1, whose shape shown in the figure is formed by etching a piezoelectric material such as quartz or the like, is provided with a base 2 having a rectangular shape, which is mounted to a package (not shown) or the like, and a pair of vibration arms 3 and 4, which extends from the base 2 in the vertical direction as viewed in the figure. Long grooves 3a and 4a are formed on the main surfaces (front and back surface) of vibration arms, and necessary driving electrodes are formed.
In the piezoelectric vibration element 1, when a driving voltage is applied via driving electrodes, the vibration arms 3 and 4 perform a flexural vibration so that their distal parts are moved closer and then apart, resulting in a signal having a given frequency being taken out.
Here, the piezoelectric vibration element 1, in which lead-out electrodes are formed at the positions indicated as numerals 5 and 6 on the base 2, is fixed to a base body such as a package or the like with adhesives 7 and 8 applied on the lead-out electrodes.
After fixing and supporting with the adhesive, cut parts 9 are formed to the base 2 so that the flexural vibration of the vibration arms is prevented from being hindered by a remaining stress caused by the differences in the linear expansion coefficient between the material of the package or the like, and the material of the piezoelectric vibration element.
In the piezoelectric vibration element 1, as a result of miniaturization, the width W1 of each of the vibration arms 3 and 4 is approximately 100 μm, the distance MW1 between them is approximately 100 μm, and the width BW1 of the base 2 is approximately 500 μm. These parts are miniaturized, so that the length BL1 of the base is accordingly shortened, thereby the piezoelectric vibration element 1 is miniaturized.
FIG. 11 is a sectional view taken along the line E-E of the vibration arm 4 in FIG. 10. In the figure, the width of the arm is W1 and exciting electrodes are not shown. In this regard, the vibration arm 3 also has the same sectional view.
The piezoelectric vibration element shows a shape shown in FIG. 12 when it is further miniaturized.
In a vibration arm 4-1 of the piezoelectric vibration element shown in the figure, the width dimension MI1 of a long groove 4a-1 for forming a driving exciting electrode becomes small when the arm width dimension is reduced to the arm width W1-1.
When quartz crystal is wet etched, etching progress is delayed in a predetermined direction due to its etching anisotropy. As a result, a protrusion or protruded part (hereinafter, called as “fin”) having a fin shape shown as indicated as numeral 4b is produced as an irregular shaped part.
If the arm width W1-1 of the vibration arm 4-1 is determined by taking the protruded dimension of a fin 4b into consideration, the arm width W1-1 is determined smaller. Accordingly, a CI (crystal impedance) value increases when a required frequency is adjusted based on the following formula: frequency (f)=k (coefficient)·W (vibration arm width)/(1(vibration arm length)×1). Namely, reducing the width of a vibration arm for miniaturization results in increasing of CI value.
As for the shape of the vibration arm 4-1 in FIG. 12, the fin 4b can be reduced when etching time for an outer shape etching of a piezoelectric vibration element is taken for a long time. As a result, field effect can be improved.
However, in this case, there is a large difference in a dimension between the thicknesses of MK1 and HK1 of walls sandwiching the long groove 4a-1 when the width dimension MI1 of the long groove 4a-1 is small.
Namely, the difference in a dimension between the thicknesses of MK1 and HK1 of walls sandwiching the long groove 4a-1 is not much improved due to a poor circulation of an etchant in a narrow groove width, and an anisotropy in etching.
In this condition, the virtual centerline C passing the center of the width dimension MI1 of the long groove 4a-1 is shifted from the gravity center position in the width direction of the vibration arm 4-1.
Accordingly, frequencies may shift to the minus side as shown in FIG. 13 when drive level characteristics of a piezoelectric vibration element are checked. As a result, a piezoelectric vibration element having favorable characteristics may not be achieved.